Mix-crystal semiconductor devices



July 5, 1966 o. G. FOLBERTH 3,259,532

MIX-CRYSTAL SEMICONDUCTOR DEVICES Original Filed Nov. 30, 1959 2Sheets-Sheet 2 K. 500 also FIG.6

United States Patent 3,259,582 MIX-CRYSTAL SEMICONDUCTOR DEVICES OttoGert Folberth, Boblingen, Germany, assignor to Siemens-SchuckertwerkeAktiengesellschaft, Berlin-Siemensstadt, Germany Griginal applicationNov. 30, 1959, Ser. No. 856,087. Divided and this application Feb. 24,1964, Ser. No.

6 Claims. (Cl. 252-623) This is a division of my application Serial No.856,087, filed November 30, 1959, now patent No. 3,140,998. My inventionrelates to improvements in mix-crystal semiconductor devices and isdescribed herein with reference to the accompanying drawings in whichFIGS. 1 and 2 show schematically two respective examples of electricalsemiconductor devices and FIGS. 3 to 6 are graphs indicative ofcharacteristic properties of various crystalline compositions used assemiconductors in such devices according to the invention.

In a more specific aspect, my invention concerns semiconductor deviceswhose active semiconductor body consists of a solid solution or mixedcrystal of two chemical compounds. Semiconductors of this type are knownfrom my Patent 2,858,275 issued October 28, 1958, and assigned to theassignee of the present invention. The semiconductors according to thepatent are formed by two binary compounds-such as InAs and InP, or GaAsand GaPwhich are both of the A B type known from Patent 2,798,989 of W.Welker, issued July 9, 1957 and assigned to the assignee of the presentinvention. There are other known semiconductor devices whose crystallinebody constitutes a solid solution or mixed crystal of polycrystalline ormonocrystalline constitution, formed by two stoichiometric compounds ofrespective elements from different groups of the periodic system, suchas those of the A B type. It should be understood that the letters A, B,C, D etc. herein used in general formulas denote different elementalsubstances from the periodicsystem groups indicated by the Romannumerals of the raised subscripts, and that the formulas indicate thestoichiometric proportion of one atom of one element to one atom of theother, unless a different atomic proportion, such as in Mg Sn, isindicated by lowered subscripts. Thus, the above-mentioned solidsolution of InAs and InP may also be written as In(As P wherein x issufiiciently larger than zero and sufiiciently smaller than unity (Ox 1) to obtain a mix-crystal substance of semiconductor propertiesappreciably different from those of the two component binary compoundsInAs and InP. Such mix-crystal semiconductors aflord obtaining propertyvalues that are intermediate those of the two component binary compoundsand thus permit tailoring the crystalline semiconductor body, as regardsits properties, to the requirements of a particular application, as ismore fully set :forth in my above-mentioned Patent 2,858,275.

It has further been found that a generally similar bridging, as toproperties, between diiferent binary semiconductor compounds of the A Btype can be obtained by substituting at least one of its two elementalcomponents in equal atomic shares by two elements from left and rightneighboring groups respectively of the periodic system. Such asubstituted A B compound may have the form C D B in which each two atomsof the A element of the original compound A i? are substituted by oneatom C from the second group and one atom D from the fourth group. Anexample of such a substituted A B compound, analogous to InAs, is thestoichiometric ternary compound CdSnAs wherein one atom of Cd and oneatom of Sn replace each two atoms of In. Such substituted A B compoundsare dealt with in the printed and published German patent applicationDAS 1,044,980.

It is an object of my invention to afford further improvements, not onlyas regards the available variety of semiconductor properties but alsorelative to thermal conductance and related or dependent properties. 7 Ihave discovered that such improvements are achieved by forming thecrystalline semiconductor of a binary semiconductor compound in solidsolution or mixed crystal with a substitute of that compound.Preferably, the crystalline body of semiconductor devices according tothe invention is a mixed crystal of the type wherein AB is anystoichiometric binary compound of two elements A, B from respectivelydifferent groups of the periodic system, and CDB is a substituted ABcompound in which each two atoms of one of the two ele ments (A) aresubstituted by one atom C and one atom D from respective groups at theleft and right respectively of the group to which the A elementappertains, it being understood that the C and D groups also differ fromthe B group. For unlimited solid solubility of the two componentcompounds AB and CDB the formula can be written as:

( x 2 1 x x 2) B wherein 0 x 1 in the sense explained above. For anyvalue of x within the stated limits, the composition of the crystal isstoichiometric in the sense that one sublattice is occupied by B atoms,whereas the other sublattice is occupied by A, C and D atoms of the sametotal number as the B atoms.

Comprehensive series of tests made with a great variety of suchstoichiometric substances have shown that, regardless of dilferences inother properties, they have in common that their thermal conductancedoes not vary monotonously from that of one component (AB) to that ofthe other (CDB but is always reduced relative to the conductance of thecompositionally nearest component, and that such reduction is greaterthan any accompanying reduction in electric properties. In many cases,the thermal conductance exhibits a very pronounced minimum. This renderssuch semiconductors particularly well applicable for semiconductor usesrequiring low thermal conductance or a low ratio of thermal to electricconductance as is the case in thermoelectric devices, thermistors orphoto-responsive resistors, aside from other uses mentioned below.

The drawing shows by way of example in FIG. 1 a thermistor or othersemiconducting resistor whose body 11 consists of a solid solution ormixed crystal according to the invention, and in FIG. 2 a thermopilewhose individual members 12 and 13 consist of substances according tothe invention having respectively different thermoforces. The members 12and 13 may consist of one and the same substance except that the members12 are doped for p-type conductance and the members 13 have n-typeconductance. The members are joined together by copper bars 14. Thedevice of FIG. 2 is suitable as a voltage generator, for example.

Before dealing with further features and examples of the invention, thefollowing explanatory remarks will be of interest.

In referring to the various groups of the periodic system, it isunderstood that the helium group of inert gases is excluded. It isfurther understood that not all of the elements in groups I through VIIcan be combined to form a binary compound constituting a solid-statesemiconductor at technologically useful temperatures, and that the bondcharacter of some of the binary, solid compounds would be too ionic forelectric semiconductor purposes.

In other words, the invention naturally involves only those elementcombinations or binary-ternary mixed crystals that do formsemiconducting compounds. In many cases, the known binary semiconductorcompounds are in the nature of intermetallic compounds formed ofrespective elements from different b-subgroups of the periodic system.The elements that form binary compounds and their ternary substitutessuitable for the purposes of the invention and mentioned in thisspecification are listed in Table No. I.

TABLE I I II III IV V VI Li B C N O Mg At St P S Cu Zn Ga Ge As Se Ag CdIn Sn Sb To An Hg Pb B1 The binary compounds to be used as one of thecomponents of a semiconducting mix-crystal for the purposes of theinvention are generally formed of respective elements in non-adjacentgroups. Thus the elements of A B compounds, for example GaAs, are spacedone group apart, and this also applies to the A E semiconductors such asPbTe, and to A B semiconductors such as Mg Sn, whereas the elements ofthe semiconducting HgTe compound are spaced three groups apart. In suchcases the analogous substitutes thus bring an element of an intermediategroup into the crystal lattice.

According to a more specific and preferred feature of my invention, thecrystalline body of a semiconductor device of reduced thermalconductance, particularly a thermoelectric device, consists of an A Bcompound in solid solution with a ternary substitute of that compound.The A B semiconductor compounds thus applicable are the nitrides,phosphides, arsenides and antimonides of boron, aluminum, gallium andindium, namely the compounds BN, AlN, GaN, InN, BP, AlP, GaP, InP, BAs,AlAs, GaAs, InAs, BSb, AlSb, GaSb and InSb. The substituted A B compoundcontained in the solid solution is preferably of the type C D B so thatthe solid-solution or mix-crystal substance contains C D B and A B inaccordance with the stoichiometric formula Such substances, used assemiconductor bodies, afford a further variation of compound-typesemiconductors as regards electric semiconductor properties as well asother physical and chemical properties. Particularly the substitution ofthe components in one phase, while generally preserving the atom latticestructure in the general sense but disturbing the lattice properties inthe range of the lattice constants, affords the possibility toconsiderably reduce the thermal lattice conductance with a relativelyslight reduction in electric conductance. Thus, the semiconductor bodiesused according to the invention permit an extremely accurate adaptationto the requirements or desiderata of the particular use intended.

This afiords a wide field of application which virtually comprises alltechnological purposes for which semi-conductor bodies are beingemployed and in which use is made of the electric semiconductorproperties of such crystalline bodies. Thus, aside from purelyelectrical uses, mix-crystal semiconductors according to the inventionare applicable for galvano-magnetic purposes (for example, Hallgenerators and electric resistors which change their ohmic resistance independence upon an applied magnetic field), various thermoelectric andphotoelectric purposes, as well as electro-optical uses, such as forelectrically controllable filters or lenses.

41 Some of the preferred mix-crystal systems of the type X/2 1-x x/2)Bare:

wherein 0 x 1 as explained above. For example, the

mix-crystals were found to have electron mobility of approximately 1300cm. /volt sec. and hole mobilities of approximately 17 cm. volt sec.;their thermal conductance being far below those of the components InAsand CdSnAs The mix-crystal system (3):(Zn In Ge )As was found to have aconspicuous minimum of the thermal lattice conductance K at a mixingratio of about x=0.75, amounting to as little as about 15% of the Kvalue of InAs (FIG. 3, curve 30). The K value decreases with increasingtemperature (FIG. 4, curves 3b to 3f). The dependence of K upontemperature (T), which follows a 1/ T law in the range from 0 to C., wassomewhat lower than for the component compounds InAs and ZnGeAs whichconstitutes another technological advantage. The measured thermallattice conductance values K of (Zn In Ge )As mix-crystals at 300 K.(degree Kelvin) were as follows:

TABLE II x K (Watt cm. Kr

1 For comparison.

On account of the negative temperature coefiicient of K the system ispreferably uitable for use in the temperature range above normal roomtemperature (above 20 C.). The high melting points in the system(InAs:936 C.;

Zn In Ge As:about 880 C.; ZnGeAs r850 C.) permit using the crystalsystem up to high temperatures in the vicinity of these melting points.Another advantage is the fact that, up to the melting point, no phasemodifications of the substances as may impair the stability will occur.By comparison, the temperature coeflicient of the compound Bi Te knownfor use at low temperatures, is negative. It is remarkable that attemperatures above normal room temperature (20 C.), the heat conductanceof the substance (14) :Zn In Ge As is even lower than that of Bi Te Theelectric conductance and the thermoforce of the solid solutions listedin Table II, as in all such cases, are greatly dependent upon doping, sothat any particular measured value of these properties is not typical assuch. However, in all cases, the above-mentioned reduction in thermallattice conductance was greater than any reduction in electricconductance so that the compositions exhibit a reduced ratio of electricto thermal conductance.

The lattice structure of the mixed-crystal system (3):(Zn In Ge )As wasfound to be cubic (Zinkblende type) for x 0.8, and tetragonal(Chalcopyrite) for x 0.8. In the system (1):(Cd In Sn )As, thedependence of the lattice conductance upon the mixing ratio is somewhatlower, although a minimum is also observed, as is apparent from thecurve 1a in FIG. 2. However, the reduction in thermal latticeconductance was more pronounced with increasing temperature as isapparrent from curves 1b through 1e in FIG. 4, in comparison with thecurves 312 through 3 relating to the system (3) (Zn In Ge )As. Thethermal conductance (lattice conductance) values of the system at 300 K.are listed below in Table III.

TABLE III x K (Watt em. K.

1 For comparison.

Further examples exhibiting an analogous reduction of thermal latticeconductance as compared with electric conductance are composed as statedin Table IV:

In the semiconductor mix-crystals used according to the invention, thenon-substituted elemental partner of the binary semiconductor compoundin the mix-crystal, particularly the non-substituted partner of an A Bcompound, may consist of two elements of the same group of the periodicsystem so that the term binary, applied to such a modification, denotesa two-group rather than a two-substance compound. Such a compound withtwo elements from one group and one element from the other,

in solid solution or mixed crystal formation with a substituted compoundof the preferred type A B C results in a composition of the type whereinx and y are greater than zero and smaller than unity: 0 (x,y) l.Examples of mix-crystals of this type are:

( x/2 1- X 2) y 1y) In the following examples, the group-III member ofthe binary (two-group) semiconductor compound is constituted by twoelements from that group:

Furthermore, a semiconductor device according to the invention may beprovided with a semiconductor body in which the elements of thesubstituted component or components, as well as such component orcomponents themselves, are partially substituted by elements of the samegroup of the periodic system. The corresponding stoichiometric crystalcompositions are of the type wherein 0 (x, y, z, t, u) 1.

6 An example of such a crystal composition formed of Zn, Cd, Ga, In, Ge,Sn, As and P, with x=0.4, y=0.2, z=0.5, t=0.5, u=0.6, is as follows:

The partial substitution of the components by elements from the samegroup of the periodic system affords, among other things, a particularlypronounced modification in heat conductance of the semiconductor body.Hence, such semiconductor bodies are likewise Well suitable forsemiconducting devices in which use is made of the thermoelectricproperties of the semiconductor body and where it is essential to have alargest feasible ratio of electric to thermal conductance.

Like the known A B compounds and the known A B substitutes, the crystalsaccording to the invention can be doped with lattice defection atomsacting as donor or acceptor. The mix-crystals therefore are readilyproducible, and can be processed, by the conventional methods, such aszone melting, for operation as extrinsic semiconductors of n-type orp-type conductance, and the known p-n junction techniques are applicablein the same manner as for the mixed crystals according to theabovementioned Patent 2,858,275. Thus, as mentioned above, thesemiconductor members 12 and 13 of the device illustrated in FIG. 2 ofthe accompanying drawings may consist of the same solid solution of an AB compound and one of its substitutes of the type C D B except that themembers 12 have n-type conductance whereas the members 13 are doped forp-type conductance.

The mix-crystals or solid solutions to be used according to theinvention can be produced by melting the two component compounds, or theindividual elements, together in stoichiometric proportions andthereafter subjecting the resulting crystalline body to zone-refining tothe extent necessary.

In cases where the composition comprises one or more elements thatevaporate at relatively low temperatures, or if the composition tends todecompose when molten, the so-called two-temperature method ispreferable. This is the case, for example, when the composition to beproduced contains phosphorus as one of its constituent elements. Thetwo-temperature method is described, for example, in the copendingapplication Serial No. 534,852, filed September 16, 1955, as well as inmy above-mentioned Patent 2,858,275, column 6, or in German Patents960,268 and 1,029,803. Mixed crystals according to the invention, whenproduced by melting the semiconductor binary compound together with itssubstituted compound, can be pulled as a crystal or monocrystal out ofthe melt in the conventional manner. In conjunction with the known zonemelting for purifying and/or homogenizing the semi-conductor crystals,the application of the above-mentioned two-temperature method is also ofadvantage.

While, as mentioned, the manufacture and further processing of mixedcrystal devices according to the invention do not require departing fromthe known methods, with the exception of the different components,number of components, or quantities involved, an example will bedescribed presently with reference to the two-temperature equipment andmethod according to Patent 2,858,275, as applied to the production of amix-crystal of the composition (33):(Zn In Ge )As, which appertains tothe mix-crystal system (3) and is related to the examples (8) to (12)according to Table II.

The starting materials are hyper-pure elemental substances inpulverulent form, namely 4000 grams In, 4570 grams Zn, 5072 grams Ge,and 13,130 grams As. The accurately weighed quantities of In, Zn, Ge areplaced in an elongated boat of graphite-coated quartz, havingsemicircular cross section, a length of 10 cm. and a width of about 1cm. The boat and its contents, together with the quantity of As, arethen sealed in a tubular quartz ampule of approximately 20 cm. length.The As quantity is placed beside the boat on the bottom of the ampule.The nmpule is then placed into a two-temperature furnace in such aposition that at first the boat and its content are heated toapproximately 900 C. There after the other end of the ampule, containingthe quan tity of As, is heated to approximately 700 C. (see FIG. 7 ofPatent 2,858,275). As a result, the content of the boat is first melted,whereas the major portion of As will sublimate at the cold end of theampule. Subsequently, the As passes through the vaporous phase into themelt. At the end of the process, substantially the entire weighedinquantities are located in the boat in form of a melt. A portion of theAs remains in the vaporous phase. Some minor precipitation, consistingessentially of Zn and As, appear on the tubular wall of the quartzampule but in negligible quantities. Thereafter the ampule is shifted inthe furnace so that gradually the entire content as sumes the lowertemperature of approximately 700 C. The desired mix-crystal then formsitself by normal freezing. The crystal is thereafter removed from theampule and may be subjected to zone melting or any desired subsequentprocessing mentioned above.

While crystal composition of A B compounds and their ternary substituteshave been found preferable for electrothermal and related or dependentpurposes because of their particularly low thermal conductance, similarresults are exhibited by other mix-crystal systems An advantage in theuse of these mixed crystals, aside from those already mentioned, is thefact that elements as heavy as Pb and Bi are suitable as components.Such use of heavy elements is another possibility of reducing thethermal conductance. A further advantage is the fact that a number ofthese crystal compounds-namely those which contain mainly Te as anelement of the sixth groupcan be readily produced by melting thecomponents or elements together in an open system. The other compoundscan be produced in accordance with the methods mentioned above,particularly the two-temperature method.

The following elements are particularly suitable for the formation ofmixed crystals of the type IV VI I V VI Elements of group Ib: Cu, Ag, AuElements of group IVb: Si, Ge, Sn, Pb Elements of group Vb: P, As, Sb,Bi Elements of group VIb: S, Se, Te

Particularly favorable among the mixed-crystal systems made and testedwere the following:

based upon the semiconducting A B compounds and their ternarysubstitutes, preferably of the type C D B Examples of this kind are:

The general formula corresponding to the above type is therefore:

( 1-x (CIX/ZDHIX/ZBVIZ) The sum of B and B is one; compare Formulas 26to 29.

Also among the mix-crystals of the type AB-CDB are the binary compoundsknown as semiconductors of the form A B" in mixture or solution with thelikewise known semiconductors of the ternary type C D B which, from theviewpoint of the present invention, can be looked upon as beingsubstituted A B compounds. Mix-crystals thus consisting of a binary A Bcompound and a ternary substitute of that compound are of the typewherein 0 x 1.

In such systems, too, the individual elemental components may be partlysubstituted by another element from the same group of the periodicsystem. In the more general case, the stoichiometric formula of such amix-crystal I IV v v EG-30 u-9, (1-x)(1-z) xt/2 2 0-0)] The diagramsshown in FIGS. 5 and 6 of the drawing are based upon measurements madewith the mix-crystal (40): (Ag Pb Bi )Te. The lattice thermalconductance at 300 K. was found to be less than 10" (watt degree cm.This is lower than the best known values of the system Bi (Te Se +Ag.For a value x of approximately 0.75 there exists a minimum of about5-10- (watt degreecmr as is apparent from FIG. 5. The heat conductancethus is three to four times lower than with the best availablemix-crystals on Bi Te basis.

With increasing temperature, the thermal conductance rapidly increases(FIG. 6) as is the case with BigTea. The heat conductance of specimenscontaining an optimum amount of doping substance is still lower thanmentioned above.

The above-mentioned doping methods as well as the conventional p-njunction techniques, such as alloying, diffusion bonding, are alsoapplicable to the mixed crystals of the type last discussed. Applicablealso is the method, known for A B compounds, of doping the crystals withthe aid of slight departures from accurate stoichiometry. Theabove-mentioned methods of melting, refining, crystal pulling,homogenizing are likewise applicable.

In most of the above-described semiconductor devices according to theinvention, the binary-group and the ternary-group components of themixed crystal are each, considered by themselves, a semiconductor. Thisis notably so with the binary-ternary mix-crystals based upon an A Bcompound and one of its ternary substitutes. However, this is notnecessarily the case with other binary-ternary mix-crystalsemiconductors according to the invention. I have found that anelectrically applicable semiconductor of this type may also be formed ofa binary compound and a ternary substitute compound of which only one,taken by itself, is a semiconductor in the strict sense, whereas theother is an appreciably ionically bonded or predominantly metalliccompound that would not be suitable for electric semiconductor purposesif used alone. Thus, in the mix-crystal (38) of SnTe and AgSbTe only theternary compound AgSbTe is a semiconductor, whereas the electricproperties of SnTe are degenerated to such an extent as to make thiscompound a metallic conductor, in contrast to such compounds as PbTe,PbSe, PbS or GeTe.

As pointed out above, the solid-solution or mixed-crystal substances tobe used for semiconductor devices according to the invention arestoichiometric. The outstanding phenomenon peculiar to thesemulti-element substances, namely a markedly greater reduction in thermalconductance, compared with electric conductance, was found to bedependent upon stoichiometry and may become overshadowed by otherphenomena if the compositions appreciably depart from stoichiometry. Itis to be understood, of course, that ideal stoichiometry cannot and neednot always be attained and that no discernable impairment of the desiredproperties may occur if such departures remain slight. In general, itcan be stated that in the above-given formulas the value of x, y, etc.,indicative of the atomic proportions, must be above 0.001, and thatdepartures below this value from the exact stoichiometry are harmless.This means that the upper limit of x must be no more than 0.999. The useof donor or acceptor atoms for imparting n-type or p-type electricconductance to the crystal does not disturb the stoichiometricproportions and has no influence upon the thermo-conductance properties,such doping substances being present in an atom percentage severaldecimal orders of magnitude 10 semiconductor body formed of (Ag Sn Bi)Te, wherein 0.001 x 1.

3. A semiconductor device comprising a mixed-crystal semiconductor bodyformed of (Ag Pb Bi )Te, wherein 0.00l x l.

4. A semiconductor device comprising a mixed-crystal semiconductor bodyformed of (Ag Pb Sb )Te, wherein 0.001 x 1.

5. A semiconductor device comprising a mixed-crystal semiconductor bodyformed of wherein 0.001 (x, y, z, t, u) 1.

6. A semiconductor device comprising a mixed-crystal semiconductor bodyformed of (Ag Pb Bi )Te, wherein x ranges from 0.7 to 0.8.

References Cited by the Examiner UNITED STATES PATENTS 4/1959 Wernick252-623 OTHER REFERENCES Wernick et al.: Constitution of the AgSbSe-AgSbTe AgBiSe AgBiTe System, J. Phys. & Che. Solids, v01. 7, November1958, pages 240-248.

TOBIAS E. LEVOW, Primary Examiner R. D. EDMONDS, Assistant Examiner.

MAURICE A. BRINDISI, Examiner.

1. A SEMICONDUCTOR DEVICE COMPRISING A MIXED-CRYSTAL SEMICONDUCTOR BODYFORMED OF (AGX/2SN(1-X)SBX/2)TE, WHEREIN 0.001$X$1.